A library file prelude.idr is automatically imported by every Idris
program, including facilities for IO, arithmetic, data structures and
various common functions. The prelude defines several arithmetic and
comparison operators, which we can use at the Idris
prompt. Evaluating things at the prompt gives an answer, and the type
of the answer:

The string.idr library allows some conversions between
primitive types and Strings, as well as some primitive operations on
Strings. We can import the library with an include declaration
(this is likely to change - Idris needs a module system at some
point.)

include"string.idr";

This library includes showInt to convert an Int to a String,
strLen to get the length of a string, among other things. We can try
these at the Idris prompt: